1. Field of the Invention
This invention relates to the metal plating of zincated aluminum and, more particularly, to providing an aluminum memory disk having magnetic properties which are substantially unchanged by heating of the disk during its manufacture.
Metal plating of aluminum is of considerable commercial interest and one application is the preparation of memory disks which are used in a variety of electronic applications such as computer and data processing systems. Aluminum is the preferred substrate for the disk although other suitable metals may be employed. Typical alloys used for memory disks are FP-3, CZ-46 and Aluminum Association Numbers 5086 and 5586. Generally, the aluminum disks are about 1.25 to 5 mm. thick and contain, by weight, about 4% to 4.9% magnesium, 0.01% to 0.4% each of copper, zinc, chromium, nickel, iron and silicon and the balance aluminum and inevitable impurities. U.S. Pat. No. 4,840,685 shows a number of other aluminum alloys suitable for disks.
In general, the disk manufacturing process involves preparing the aluminum surface, e.g., zincating, followed by depositing a paramagnetic sublayer of electroless nickel phosphorus (ENP) which is used as the base for a thin layer of ferromagnetic material, i.e., Co, CoNiCr, etc. usually applied by sputtering. As plated ENP deposits in excess of about 9% by weight phosphorus are paramagnetic but these deposits lose their amorphous structure and become ferromagnetic above about 290.degree. C. Elevated temperatures on the order of 310.degree. C. can be reached during the sputtering process and at increasing temperatures even more thermally stable ENP deposits are required. By "ENP" is meant herein to be a electroless nickel deposit containing greater than about 9% by weight phosphorus.
The memory disk industry requires that the ENP deposits remain substantially nonmagnetic, e.g., less than 5 gauss and preferably at its original level of less than 2 gauss because if the deposit is ferromagnetic it would interfere with the read/write modes by diluting signal and increasing noise levels.
This requirement has received attention in the industry and a number of articles have been written addressing this matter. M. Schwartz and G. Mallory, "Effect of heat-treatments on magnetic properties of electroless nickel alloys, J. Electrochem Soc.: Vol. 123, No. 5, pages 606-614, May 1976; Saad K. Doss and P. B. P. Phipps, "Process for the preparation of electroless nickel with superior thermal stability" Plating and Surface Finishing, April 1985, pages 64-67; Koiwa, Usoda, Yamada and Osaka, "Effect of heat treatment on properties of electroless deposited nickel Ni-Mo-P alloy films" J. Electrochemical Society, Vol. 135, No. 3, March 1988, pages 718-726; and J. B. Haydu, E. F. Yarkosky, S. Kawashima, H. Nakao, H. Iwasawa "Magnetic induction in EN deposits for aluminum memory disk", presented at the Second Int. Symposium on Magnetic Materials, Processor and Devices sponsored by IBM, Oct. 13-18, 1991, Phoenix, Ariz.
In the Doss and Philips article it is demonstrated that the introduction of boron in ENP films generally enhanced their paramagnetic thermal stability. In general, the annealing temperatures at which the ENP films became ferromagnetic were found to be higher as the boron content of the film was increased. Boron was introduced in the deposit by adding dimethylamine borane (DMAB) to the electroless nickel plating solution. Typical deposits contained 87 to 88% by weight nickel, 11 to 12% phosphorus, about 1% copper and up to 1% boron. Similar results for boron containing ENP deposits are shown in U.S. Pat. No. 3,953,654.
It is an object of the present invention to provide a method for preparing metal plated aluminum substrate articles wherein retention of their original magnetic properties after exposure to elevated temperatures is enhanced.
It is a further object of the present invention to provide metal plated aluminum substrate articles such as memory disks wherein retention of the original paramagnetic properties of the electroless metal plating, e.g., ENP, after exposure to elevated temperatures is enhanced.
Other objects and advantages will become apparent from the following detailed description.
For convenience, the following description will be directed to double zincated aluminum substrates and electroless nickel plating baths although it will be clear to those skilled in the art that other suitable protective coatings and metal plating baths may be employed.